1. Field of the Invention
The present invention relates to a CMOS image sensor, and more particularly to a CMOS image sensor and method for manufacturing the same, in which a diffusion region of a photo diode is disposed apart from an isolation region, and thereby a dark current is reduced.
2. Description of the Prior Art
In general, an image sensor is a semiconductor device for converting an optical image into electrical signals, and is generally classified into a charge coupled device (CCD) and a complementary MOS (CMOS) image sensor.
A CCD generally includes a plurality of MOS capacitors and each MOS capacitor is adjacently disposed to each other. Charge carriers are stored on one of the MOS capacitors and then transferred to another MOS capacitor next to the MOS capacitor with the charge carriers stored therein. A CCD has various disadvantages, such as complicated drive mode, high power consumption, complicated manufacturing process, i.e., a large number of processing steps, and so forth. Additionally, a CCD has a disadvantage in that it is difficult to make a compact-size product, due to the difficulty in integrating various circuits such as a controlling circuit, a signal processing circuit, analog/digital converting circuit and so on into a single chip.
Currently, as a next generation image sensor for overcoming the disadvantages of a CCD, attention is attracted to CMOS image sensors. A CMOS image sensor is a device employing a switching mode of forming a photo diode and a MOS transistor in each unit pixel on a semiconductor substrate using CMOS technologies. A CMOS image sensor generally includes a controlling circuit, a signal processing circuit, and so on as a periphery circuit, and sequentially detects outputs of each unit pixel by means of the MOS transistors. Thus, with the photo diode and MOS transistor formed within each unit pixel, a CMOS image sensor sequentially detects electrical signals of each unit pixel in a switching mode to realize an image.
A CMOS image sensor has advantages such as low power consumption, simple manufacturing process, i.e., small number of processing steps, and so on. In addition, a CMOS image sensor has an advantage in that the product is made compact by integration of a controlling circuit, a signal processing circuit, an analog/digital converting circuit, etc. into a single chip. Therefore, CMOS image sensors are presently broadly used in various applications, such as digital still cameras, digital video cameras, and so forth.
FIG. 1 shows a circuit diagram for a unit pixel of a conventional CMOS image sensor. As shown in FIG. 1, a unit pixel 100 of the CMOS image sensor includes a photo diode 110 as a photoelectric transformation section, and four transistors, including a transfer transistor 120, a reset transistor 130, a drive transistor 140, and a select transistor 150. An output terminal OUT of the unit pixel 100 is connected with a load transistor 160. Herein, a reference label FD indicates a floating diffusion region, a reference label Tx indicates gate voltage of the transfer transistor 120, a reference label Rx indicates gate voltage of the reset transistor 130, a reference label Dx indicates gate voltage of the drive transistor 140, and a reference label Sx indicates gate voltage of the select transistor 150.
FIG. 2 shows a layout of a unit pixel of the conventional CMOS image sensor. As shown in FIG. 2, in the unit pixel 100, an active region is a region defined by a bold solid line and an isolation region is a region outside the active region in which an isolation layer (not shown) is formed. The gates 123, 133, 143 and 153, respectively of the transfer transistor 120, reset transistor 130, drive transistor 140 and select transistor 150, are disposed as shown in FIG. 2. The reference label FD indicates a floating diffusion region, and the reference label PD indicates a portion of the photo diode 110.
FIG. 3 is a structural cross-sectional view showing the photo diode portion of the unit pixel taken along a line A—A of FIG. 2. As shown in FIG. 3, a P− type epitaxial layer 11 is formed on a P++ type semiconductor substrate 10, wherein P++ indicates a heavily doped region. To define an active region of the semiconductor substrate 10, an isolation region 13 is formed in a portion of the epitaxial layer 11. An n− type diffusion region 111 and a P0 type diffusion region 113 of the photo diode 110 are formed in a portion of the epitaxial layer 11, the P0 type diffusion region 113 being positioned on the n− type diffusion region 111, wherein n− indicates a low doping of impurities, and P0 indicates a medium doping of impurities.
The conventional CMOS image sensor 100 with such a structure has disadvantages such as degradations of the performance and electric charge storing capacity, due to an increase of dark current, which is generated by electrons being transferred to the floating diffusion region from the photo diode 110 when no light is received by the photo diode 110.
It has been reported that dark current has been caused generally from various kinds of defects, dangling bonds, and so forth, in a portion adjacent to the surface of the semiconductor substrate 10, a boundary portion of the isolation region 13 and the P0 type diffusion region 113, a boundary portion of the isolation region 13 and the n− type diffusion region 111, a boundary portion of the P0 type diffusion region 113 and the n− type diffusion region 111, the P0 type diffusion region 113 and the n− type diffusion region 111.
By using both the P0 type diffusion region 113 and the n− type diffusion region 111 for the photo diode, the conventional CMOS image sensor 100 has reduced dark current generated in the portion adjacent to the surface of a silicon substrate.
However, the conventional CMOS image sensor 100 has been greatly affected by dark current generated at the boundary portions of the isolation region 13 and the P0 type diffusion region 113, and in the P0 type diffusion region 113 and the n− type diffusion region 111.
Particularly, as shown in FIG. 3, when a photoresist pattern (not shown) used as a mask layer during ion implantation for forming the n− type diffusion region 111 and the P0 type diffusion region 113 is formed on the semiconductor substrate 10, the whole active region for the photo diode 110 is exposed. When impurities for the n− type diffusion region 111 and the P0 type diffusion region 113 are ion-implanted in the active region of the photo diode 100, those impurities are also ion-implanted into the boundary portion between the active region and the isolation region 13 of the photo diode 110.
Thus, damages are caused by the ion implantation of impurities at the boundary portion between the n−/P0 type diffusion regions 111 and 113 and the isolation region 13, further generating defects. The defects may cause a generation of electron or hole carriers, and provide recombination centers for the electrons and the holes, thereby increasing the leakage current of the photo diode and the dark current of the CMOS image sensor.
As described above, a conventional CMOS image sensor has such a structure that, when forming the diffusion region of the photo diode, impurities are also ion-implanted in the boundary portion between the isolation region and the active region for the photo diode, thereby increasing the dark current of the photo diode. Also, with such a structure, it is difficult to maintain uniform device characteristics among the pixels of the image sensor. Therefore, the performance of the conventional image sensor is generally not satisfactory.
Pending Korean Patent Publication Nos. 2003-42303 and 2003-42308 disclose a method for reducing dark current of CMOS image sensor by implanting impurities into an active region for forming a photo diode. However, they do not provide a solution to restrict an increase of dark current by preventing impurities from being ion-implanted into the boundary portion between an isolation region and an active region for a photo diode.
Also, U.S. Pat. Nos. 6,486,521 and 6,462,365, to Omnivision Technologies Inc., both entitled “Active Pixel Having Reduced Dark Current in a CMOS Image Sensor,” disclose a method for restricting an increase of dark current due to dangling bonds at the surface of a photo diode, by forming a transparent insulating layer such as an oxide layer on the surface of the photo diode as a passivation layer. However, neither do these patents provide a solution to restrict an increase of dark current by preventing impurities from being ion-implanted into the boundary portion between the isolation region and the active region for the photo diode.